Torsional forces are exerted upon the bed or frame of a semitrailer whenever road surface irregularities are encountered in one quadrant of the tractor/trailer assembly, and the irregularity cannot be absorbed entirely by the suspension system in the affected quadrant. If the bump or depression is sufficiently extensive, an upward or downward force will be exerted on that portion of the vehicle's frame to which the overloaded suspension components are attached. Since such an upward or downward movement is substantially perpendicular and lateral to the longitudinal axis of the tractor/semitrailer rig, it creates a twisting or torsional force in the frame, unless offset by some countermovement in another area. For the most part, the fifth wheel device by which the semitrailer is connected to the truck tractor cannot absorb twisting or torsional forces, and thus transmits such forces between the truck and the trailer.
Where, as here, the highway transporter is designed to carry one or more prefabricated building modules, these torsional forces must be stopped before they reach the cargo. Each module is a relatively wide, box-like structure that extends laterally somewhat beyond the sides of the trailer bed, and is thus too large to disperse or fractionate torsional forces without being damaged. Each module is provided with a number of relatively rigid and/or precisely positioned structures, such as brazed pipe connections, finished ceilings and walls, hung windows and doors, and a wide assortment of trim pieces. None of these components would be protected adequately with padding if they were twisted.
There is ample opportunity for torsional forces to arise, due to the course which the tractor/semitrailer assembly must take with the prefabricated building modules. Ordinarily, there are considerable distances between the factories where the modules are manufactured and the construction sites where the modules are assembled together. As a result, the usual road hazards like potholes, soft shoulders, debris and uneven grades and driveway aprons, can cause substantial twisting forces to be exerted on the semitrailer. In addition, the tractor/trailer is frequently driven over rough terrain at the construction site in order to unload the modules directly onto their foundations. Since the module must be anchored securely to the trailer during transit, any twisting motion in the trailer bed is likely to be transmitted to the module, unless diverted or eliminated altogether.
In the past, it was proven to be economically advantageous to build semitrailers that could accommodate two of the prefabricated building modules. These early transporters were provided with crossing steel reinforcing beams extending diagonally between the sides of the trailer bed, and with other reinforcing members that prevented the semitrailer from twisting under almost any road conditions. They were designed, in effect, to transmit torsional forces generated in one quadrant of the tractor/trailer directly to the axle/wheel suspension components at the other corners of the rig. While they functioned adequately for a period of time, their weight created an undesirable load on the truck tractor, and their rigidity created control problems and excessive strain on the fifth wheel device.
The most serious drawback, however, to the early transporters was their inability to accommodate an improved braking system. It was found that in order to provide more braking power, a new air brake and rear axle assembly had to be positioned more rearwardly on the semitrailers than the outmoded equipment had been. Unfortunately, when the earlier trailer was provided with a longer wheelbase and then loaded with two modules, the central portion of the trailer bed sagged. Due to the height of each module and the desirability of providing an inclined bed to urge the modules towards the middle of the trailer, the central portion of the bed was positioned closer to the ground than the opposite ends of the trailer. Accordingly, little or no sag in the central region could be tolerated. In an effort to strengthen this area, additional reinforcement was added. However, by the time it was adequately supported against both sagging and torsional movement, the earlier transporter was too heavy to meet axle weight limits.
By then, it was clear that an entirely new highway transporter was needed. It occurred to the present inventors that, instead of designing another rigid trailer that could not twist under torsional forces, they would try to devise a flexible trailer provided with means for dissipating the torsional movement generated therein before it reached the cargo. As a result, the present invention was conceived.